In a semiconductor manufacturing process, for example, during execution of an etching process, a film thickness of a semiconductor film on a substrate changes with time to decrease. During execution of a thin-film formation process, a film thickness of a semiconductor film changes with time to increase. In such a semiconductor process, for process control such as detection of an end point of the process, etc., in-situ measurement of a temporal change in film thickness of the semiconductor film is necessary.
As such a measurement method for the film thickness of a semiconductor film, there is used a method in which a semiconductor film is irradiated with measurement light with a predetermined wavelength, and interfering light generated by interference of reflected light from the upper surface and reflected light from the lower surface of the semiconductor film is detected. In this method, when the film thickness of the semiconductor film changes, an optical path length difference between the reflected light from the upper surface and the reflected light from the lower surface changes. Therefore, by utilizing a temporal change in detected intensity (interference intensity) of the interfering light corresponding to the change in optical path length difference, film thicknesses of the semiconductor film at each time point can be measured (for example, refer to Patent Documents 1 to 4).